Over the past decade, fiber optic sensors have received attention in the application of magnetic field sensing and current sensing. Fiber optic current sensors are particularly advantageous over iron-core current transformers, since fiber optic sensors are non-conductive and light weight. Furthermore, fiber optic sensors also do not exhibit hysteresis and provide a much larger dynamic range and frequency response.
Fiber optic current sensors work on the principle of the Faraday effect. Current flowing in a wire induces a magnetic field which, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law, stated as: EQU I=.intg.oH.multidot.dL
where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, .DELTA..phi., through which the plane of polarization of light rotates in the presence of a magnetic field is given by: EQU .DELTA..phi.=V.intg.H.multidot.dL
where V is the Verdet constant of the fiber glass. The sensing optical fiber performs the line integral of the magnetic field along its path which is proportional to the current in the wire when that path closes on itself. Thus, .DELTA..phi.=VNI, where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current may be measured by injecting light with a well defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region.
In related U.S. Pat. No. 5,644,397 entitled Fiber Optic Interferometric Current and Magnetic Field Sensor, issued on Jul. 1, 1997 to James N. Blake (Hereinafter "Blake"), in-line and loop fiber optic sensors for measuring current and magnetic fields are taught. Blake is incorporated herein by reference. Blake teaches splitting the light beam into light traveling on the first and second principle eigen axes, the use of a birefringence modulator to apply a waveform or waveforms to birefringent modulate the light beam, and further the use of a quarter waveplate set at 45.degree. to the principle axes of the fiber to convert orthogonally linearly polarized light to counter-rotating circularly polarized light prior to entering the sensing region. Upon reflection at the end of the fiber, the sense of rotation of the two light waves are reversed and the light waves travel back through the sensing region, are converted back to linearly polarized light, and are propagated back to a photodetector. The two light waves therefore undergo reciprocal paths and the same polarization evolution through the optical circuit. Blake is incorporated herein by reference.
The fiber optic sensors taught by Blake overcame many disadvantages associated with conventional all fiber sensors. However, the sensor and sensing method still suffers from a particularly exacerbating problem which affects the accuracy of the sensor. To have a very accurate measurement, the optical components, particularly the quarter waveplate, must be perfect and not be affected by external stresses such as temperature variations and mechanical disturbances. It is well recognized that perfect or nearly perfect quarter waveplates are difficult and very costly to design and manufacture to achieve accurate sensing required by certain applications.